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My son asks the above (if not in quite these words), and I am embarrassed to realize that I do not know. Can gravity, for example, or the strong or weak forces ever be repulsive? How/when?

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    $\begingroup$ You mean, other than electrostatics? You know, that whole "opposite charges attract, like charges repel" business. $\endgroup$ – Emilio Pisanty May 6 '18 at 14:55
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Famously, magnets have North and South poles, and like repels like. You can think of these as positive and negative "magnetic charges". Similarly, electrical charges obey the like-repels-like rule (e.g. two electrons are both negatively charged and repel each other).

The strong and weak nuclear forces can be thought of as more complicated cousins of electromagnetism, with multiple charges that still come in positive and negative. (For example, you can think of quarks as having a positive charge called a color, antiquarks as having a negative charge called an anticolor, and gluons as having each, like the poles of a magnet.) The details are more complicated, but yes, like still repels like.

When we look at gravity, we find positive "charges" called masses that attract each other. Why is this case different? It turns out to be due to EM and nuclear forces having spin-1 carriers and gravity's hypothetical carriers being spin-2, but that's probably overkill for this question.

Could gravity repel? It would require opposite charges, i.e. positive and negative, and where would you get negative mass from? The cosmological constant $\Lambda$ that accelerates the expansion of the universe can be thought of as a negative contribution to the universe's density, resulting in a repulsive effect.

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  • $\begingroup$ Related: Can a third type of electrical charge exist? $\endgroup$ – Emilio Pisanty May 6 '18 at 16:35
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    $\begingroup$ Whether antimatter falls down (i.e., is repelled gravitationally by normal matter) is an ongoing ALPHA experiment at the LHC. The early results are not very tight (antihydrogen is not gravitationally repelled as if its gravitational mass were less than $-65$-times its inertial mass -- so we need to improve that result about 100-times to really resolve the question). $\endgroup$ – Eric Towers May 6 '18 at 18:21
  • $\begingroup$ @EricTowers Maybe one day they'll prove that example of repulsion. If they do, it'll trip up lots of ideas: en.wikipedia.org/wiki/… $\endgroup$ – J.G. May 6 '18 at 18:43
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    $\begingroup$ Gravity being different isn't "due to" gravitons having -2 spin. If gravitons even exist they have to be -2 because that's how gravity is observed to work. $\endgroup$ – Stop Harming Monica May 7 '18 at 16:16
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    $\begingroup$ Alternatively, gravity is different because it doesn't involve quantum fields at all and is just temporal-spacial geometry, or because gravity is mediated by closed rather than open strings. We don't "know" gravitons are spin-2 - we don't even know they exist. It's just some maths we made up to try to describe what happens. $\endgroup$ – Stop Harming Monica May 7 '18 at 16:28
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The strong nuclear force repels two particles when the distance between them is less than about 0.5fm. If this wasn't the case, then one implication of that would be that nuclei would implode in on themselves. After 0.5fm, it attracts, but at about 4fm it stops attracting.

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    $\begingroup$ Note that this is not talking about the "strong nuclear force" of QCD that keeps quarks bound into hadrons; asymptotic freedom of QCD means that the strength of the strong force falls to zero at short distances. The force you're talking about is rather the "residual strong force" that keeps nucleons together in the nucleus, which is mediated by meson-exchange rather than gluon-exchange. $\endgroup$ – probably_someone May 6 '18 at 19:03
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The simplest example is electrostatic force: positive charges repel.

More complicated, is that the gravitational "force" can also be repulsive in the case of a fluid with negative pressure. This is counter-intuitive, because we would think positive pressure pushes things apart, and thus negative pressure pulls them together. But in relativity, pressure is a form of energy, and thus gravity. so negative pressure gives negative gravity, and thus pushes things apart.

Ah, but you might ask, what in the world has negative pressure? Amazingly enough, it seems the entire vacuum of the Universe has negative pressure. We call it the cosmological constant, or more generally, quintessence, and we have known for about 20 years now that it is making the universe expand faster and faster, due to repulsive gravity.

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  • $\begingroup$ While risking sounding like an uneducated fool, I wonder if we can "bottle" that stuff up and make a working antigrav? $\endgroup$ – htmlcoderexe Jan 15 at 13:16
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Gravity is closed field lines. It cannot be ever repulsive. Yes, you can generate a force stronger then it and overcome it but there is no such thing as true anti-gravity.

Since the official physics is clueless currently about what actually generates the strong or weak forces, it's sufficient to say that anything on this subject would have no actual bases.

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  • $\begingroup$ What about in the theoretical case of negative mass? $\endgroup$ – Michael Richardson May 7 '18 at 15:57
  • $\begingroup$ No such thing for the exact same reason. The positron for example, has the exact same mass as the electron. Nothing negative in all particle-related physics. $\endgroup$ – Overmind May 8 '18 at 5:35

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